Grazing incidence small angle X-ray scattering (GISAXS) methods are frequently very successfully utilized for morphological investigations of thin polymer blend films used in organic photovoltaics. However, conventional GISAXS does no longer provide material sensitivity when the blend consists of more than two components due to the contrast conditions. In case of multi-component blends the use of grazing incidence resonant soft X-ray scattering (GI-RSoXS) can overcome such problems. In this work we exemplarily apply GI-RSoXS to investigate simultaneously the vertical and lateral morphology of polymer-fullerene bulk heterojunction layers made from poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl]] (PCPDTBT) and [6,6]-penyl-C 61 butyric acid methyl ester (PCBM) with added photosensitizer perylene diimide (PDI) with and without use of the solvent additive 1,8-octanedithiol (ODT). The investigation reveals that films without solvent additive ODT tend to show only vertical phase separation while films prepared with solvent additive also phase separate laterally. PDI seems to assist lateral phase separation between PCPDTBT and PCBM by expelling PCBM from the amorphous polymer matrix.(C) 2016 Elsevier Ltd. All rights reserved.